CuNi Nanoalloys with Tunable Composition and Oxygen Defects for the Enhancement of the Oxygen Evolution Reaction**

Abstract Colloidal synthesis is an excellent tool for the study of cooperative effects in nanoalloys. In this work, bimetallic CuNi nanoparticles of defined size and composition are fully characterized and tested for the oxygen evolution reaction. Copper addition to nickel leads to modifications in the structural and electronic properties, showing a higher concentration of surface oxygen defects and formation of active Ni 3+ sites under reaction conditions. The ratio O V /O L between oxygen vacancies and lattice oxygen shows a clear correlation with the overpotential, being an excellent descriptor of the electrocatalytic activity. This is attributed to modifications in the crystalline structure, leading to lattice strain and grain size effects. Bimetallic Cu 50 Ni 50 NP showed the lowest overpotential (318 mV vs RHE), low Tafel slope (63.9 mV dec −1 ), and excellent stability. This work unravels the relative concentration between oxygen defects and lattice oxygen (O V /O L ) as an excellent descriptor of the catalytic activity of bimetallic precatalysts.